Light has long been proposed to have a stimulatory effect on a range of biological functions in humans, including increased feelings of activation, such as improved alertness or ability to perform. The mechanisms underlying how light stimulates these neurobiological systems remain to be elucidated. Although a sizeable literature exists on how different properties of white light activate neurological systems, such as resetting the circadian pacemaker or inhibiting the sleep-promoting hormone melatonin, only a few studies exist describing direct effects of white and colored light on arousal mechanisms, and their findings are ambiguous. We propose to investigate the effects of different colors of light on human physiology, and in particular test the claims that specific colors of light preferentially stimulate neurobiological, physiological and endocrinological systems. Using classical photobiological techniques, we will construct action spectra for the effects of different colors of light on a range of non-image forming responses in humans. We will test the hypotheses that: 1) light-induced activation of the neurobiological sleep-wake regulatory system, as indicated by increased alertness, faster reaction time, suppression of EEG alpha activity, microsleeps and slow rolling eye movements, and suppression of pineal melatonin, is most sensitive to retinal exposure to short-wavelength blue light (460 nm) compared to equal photons of other colors of visible light; 2) light-induced activation of autonomic and hypothalamic-pituitary-adrenal axis measures of arousal, as indicated by increased heart rate variability, core body temperature, blood pressure, respiration rate, plasma cortisol levels and urinary catecholamines, is most sensitive to exposure to short wavelength blue light (460 nm) compared to equal photons of other colors; 3) phase shifts of the human circadian pacemaker, as assessed by changes in temperature, melatonin and cortisol rhythms, are most sensitive to exposure to short-wavelength blue light (460 nm) compared to equal photons of other colors. The resultant action spectra will help to identify the photoreceptor mechanism(s) by which light activates arousal and circadian resetting, these non-image forming physiological responses and enable us to distinguish between major candidate photoreceptive mechanisms, including potential novel photoreceptor systems, that might mediate such responses.